Determination Of Barrier Height Research Articles

Effects of thermal excitation and quantum-mechanical transmission on photothreshold determination of Schottky barrier height

Quantum-mechanical electron transmission effects on photothreshold determination in Schottky barriers with high electric fields are shown to produce a strong effect on apparent photothresholds without a concomitant large change in shape of the photoresponse curve. Quantum-mechanical electron transmission probabilities are determined by numerical integration of the Schroedinger equation for a Schottky barrier model which includes Thomas-Fermi shielding in the metal, image force lowering in the semiconductor, and conservation of transverse momentum at the metal-semiconductor interface. Results are presented for <111> n-type Si and n-type GaAs. In the case of Si, conservation of transverse momentum at the interface produces a discontinuity in the electron kinetic energy associated with motion normal to the interface but does not greatly affect the transmission probability. Using these results, the difference between the true barrier height, qΦ B , and the apparent barrier height, qΦ A , is calculated as a function of temperature, doping, and electric field at the metal-semiconductor interface. qΦ A is defined as the intercept on the hv axis of the slope of a plot of (photocurrent per absorbed photon) 1 2 versus quantum energy, hv. Effects of barrier curvature due to doping are negligible for impurity densities below about 10 18 cm −3 for Si and 10 17 cm −3 for GaAs. Values of q( Φ B - Φ A ) in excess of 50 meV occur in many cases. The corrections derived are used to reinterpret previously reported measurements of the dependence of Schottky barrier height on temperature and impurity concentration.

Admittance spectroscopy of impurity levels in Schottky barriers

Measurements of the complex admittance of Schottky-barrier diodes as a function of temperature provide a spectroscopy of deep trapping levels. The measurement conditions are usually close to thermal equilibrium, thus assuring the validity of equilibrium occupation probabilities. An exact (i.e., to arbitrary accuracy) solution for the junction admittance is given. The problem is reduced to solution of a simple initial-value problem, the final integration being carried out by computer. It is shown that the dispersion in capacitance due to slow trapping levels can lead to serious errors in estimates of junction doping concentrations and barrier height determinations. Examples are given for p-type ZnTe where Shockley-Read-Hall (SRH) centers are dominant and for n-type CdTe and Cd1−xZnTe where double-acceptor centers are dominant. For junctions formed on CdTe : Ga it is shown that one of the steps in capacitance observed in a thermally stimulated capacitance survey is actually a high-frequency response step and is not due to slow emission from a trapping center.

On the barriers to planarity and the isotope effect in cyclobutane and cyclobutane- d8

The infrared and Raman data on the ring-puckering vibration in cyclobutane and cyclobutane- d 8 have been reexamined including the coordinate dependence of the reduced mass in the Hamiltonian. This was done for the purpose of estimating the importance of these small terms in the determination of barrier heights for four-membered rings and also on the determination of the dihedral angle corresponding to the potential minimum. The conclusions reached are that there is an isotopic dependence of the barriers to planarity in cyclobutane and cyclobutane- d 8 yielding a difference of ∼14 cm −1, but the precise value of the difference in barrier heights is ill determined. The higher-order kinetic energy terms in the Hamiltonian can account for a spread of ∼3 cm −1 in each of the barriers derived for cyclobutane and cyclobutane- d 8, depending on the details of the model used for the vibration, but not a difference of 14 cm −1, which undoubtedly indicates the effects of coupling with other vibrational modes. It is also found that the derived values of the dihedral angles are quite sensitive to the details of the vibrational model, in fact, much more so than to the uncertainties in the bond distances and bond angles. A relationship between the potential constants derived for cyclobutane and cyclobutane- d 8 assuming an effective constant reduced mass and those derived for a semirigid model is demonstrated.

Impurity recombination radiation in GaAs after surface doping

A change in the spectra of impurity recombination radiation from an etched surface of GaAs coated with Ni and Cu is investigated. It is found that using the electrochemical method of coating Ni, a new band of recombination radiation with the maximum near (1.27 to 1.33) eV is obtained under photoexcitation near the surface. The investigations of the surface treatment conditions, giving rise to the new band together with the estimate of the diffusion length of Ni into the bulk of GaAs, the determination of the potential barrier height on the NiGaAs interface, and the estimate of the change of occupation of deep centres in the space-charge region at the surface lead to the conclusion about arising of surface radiative recombination centres in GaAs as a result of the electrochemical coating of the surface with nickel. New centres of radiative recombination are likely to be complexes involving Ni and a structure which seem to be the defect. It is shown that coating of the surface of GaAs with copper with the doping methods used does not create surface centres of radiative recombination. [Russian Text Ignored].

Photodetection by light-induced barrier modulation in Cu-diffused Au–CdS diodes

Modified Schottky barriers of the type Au–CdS : Cu were prepared by diffusing Cu to a depth of 0.1–0.2 μm into single crystals of CdS, prior to evaporating rectifying Au contacts. The electronic and photoelectronic properties of these junctions are adequately described by a simple model in which the Cu acts as an ionized acceptor, resulting in a ``humped'' potential barrier between the Au and the bulk CdS. Hole trapping by the acceptors under band-gap illumination reduces their ionization and, consequently, the hump height. The forward current density in the dark and under illumination can be accurately described by a thermionic emission model as J=A*T2 exp {—e[VB+VH(0)]/kT exp}(eV/βkT), where the hump potential VH (0) is light sensitive and where β (1&amp;lt;β&amp;lt;1.35) arises from a weak voltage dependence of the barrier height. Barrier height determinations, in the dark and under illumination with 5000-Å light, by means of internal photoemission, thermal activation energy, and J-V measurements unequivocally show that under saturating light conditions the hump in the potential barrier is reduced by 0.23±0.03 V. This barrier mechanism of photoconductive response results in steady-state electron/photon gains in excess of 106 at light intensities lower that 5×1011 photons cm−2 sec−1, where response times exceed 1 sec.

Microwave spectrum, barrier to internal rotation, structure, and dipole moment of methane selenol

The microwave spectra of the ground state and excited torsional states of several isotopic species of methane selenol have been observed. The barrier hindering internal torsion has been determined to be 1001±50 cal/mole. The structural parameters are C–Se= 1.959 Å, Se–H= 1.473 Å, C–H= 1.088 Å, ∠C–Se–H= 95.45°, ∠H–C–H= 110.00°, with the symmetry axis of the methyl group tilted 1.5° away from the C–Se internuclear line. The dipole moment components are μA=1.253 D, μB=0.314 D, μtotal=1.291 D. The implications of this barrier height determination for theories on the orgin of the barrier are discussed.

Barrierenhöhen von metall-halbleiterkontakten

This paper presents the results of an investigation on the barrier-hieghts of Schottky barriers, produced by evaporation of gold, tantalum and tungsten on silicon 111-surfaces. A theory is outlined which allows for a quantitative determination of barrier heights. Values thus obtained are in good agreement with the experimental results. This theory is based on the assumption that the surfaces state distribution of the silicon-interface at the Schottky junction is equal to the surface state distribution of a free silicon surface.

Generalized Thermal J-V Characteristic for the Electric Tunnel Effect

The thermal J-V characteristic for a tunnel junction is derived in terms of a generalized theory. The resulting functional form of the equations is similar to that of Stratton; however, in the present formulation, the physical parameters of the junction appear explicitly, and their effect upon the thermal characteristic is readily appreciated. In Stratton's work, the physical constants appear in the integrand of integral that can be solved only numerically. The theory is applied to symmetric and asymmetric junctions. For the symmetric case, it is shown that, at a given temperature, the percentage change Ĵ in the high-temperature thermal component of current from the low-temperature value increases initially with increasing voltage bias up to a maximum peak, and thereafter decreases rapidly. The voltage bias at which the component of thermal current maxima occurs is equal to the interfacial barrier height and, as such, permits what is probably the most accurate method of barrier height determination. Similar results are obtained for the asymmetric barrier; however, in this case, Ĵ depends upon the polarity of the voltage bias for V&amp;gt;φ1, and two Ĵ maxima occur at voltages corresponding to the two distinct interfacial barrier heights φ1 and φ2.

Determination of Barrier Heights from Low-Temperature Heat-Capacity Data

A method is presented by which barrier heights, in potential functions hindering the internal rotation of methyl groups, or spectroscopically inactive vibrational frequencies, can be obtained solely from thermal data for the solid phase. The observed heat capacities are analyzed in terms of contributions from the lattice vibrations, the internal vibrations, the expansion of the lattice, and the internal rotation. At low temperatures, the hindered rotation can be treated as torsional oscillation with a frequency related to the barrier height. It is shown that barrier heights derived by this method are in accord with those estimated from comparison of gas-phase entropies, but can be determined with greater precision than such values.

Determination of the Semiconductor Surface Potential under a Metal Contact

It is shown that through the use of the infrared absorption technique to measure the added carrier density in a semiconductor bulk adjacent to a metal contact coupled with simultaneous measurement of the floating potential of the contact, a reliable determination of the semiconductor surface barrier height under the metal contact can be obtained.

A Study of Carrier Injecting Properties of Emitter Contacts and Light Spots at Normal and Moderately Elevated Temperatures

The method of determining the injection efficiency γ of an emitter contact (described previously by Shockley, Pearson and Haynes) is discussed and some of the difficulties of the method and the interpretation of experimental results are described. The effects of a fine spot of white light when used as an emitter and hence as a conductivity modulator are investigated and an equivalent minority carrier current for a given optical assembly can be determined. This procedure suggests an experimental method for the determination of γ as a function of temperature. A simple theory for γ in terms of forbidden energy gap, depth of Fermi level, and barrier height is given and applied to the determination of barrier heights at room temperature, at elevated temperatures, and when an emitter contact on germanium is illuminated. Theoretical curves relating γ and are presented for Ge and Si of various impurity concentrations. The experiments carried out at elevated temperatures suggest that the surface states on germanium lie at the top of the filled band and that their density is of order 1010-1011 cm-2.